1. Field of the Invention
The present invention relates generally to compositions for use in making golf ball cores. In particular, the invention relates to such golf ball cores having a difference in hardness between the core's outer surface and the core's center point. The present invention also relates to methods for manufacturing these golf ball cores.
2. Description of Related Art
Golf balls generally include a core and at least one cover layer surrounding the core. Material characteristics of the compositions used in the core, and the resulting mechanical properties of the core, are important in determining the golf ball's performance. For example, the core's composition affects the golf ball's coefficient of restitution (C.O.R.), i.e., the ratio of the ball's post-impact speed to pre-impact speed. The C.O.R. affects the ball's speed and distance when hit. The core's composition also affects the ball's compression, i.e., a measure of the deflection of the ball when a standard force is applied to the ball. Cores exhibiting low compression provide for improved ball feel, but also tend to exhibit reduced C.O.R., which results in reduced ball flight distance.
Golf ball cores generally incorporate polybutadiene rubbers cross-linked with sulfur compounds, or peroxides, and a metal salt of an acrylate, such as zinc diacrylate (“ZDA”) or zinc dimethacrylate (“ZDMA”). These compositions provide for improved properties; however, despite years of continual improvements in rubber core formulations, ideal properties have not yet been achieved for golf balls. Increasing the loading levels of sulfur compounds, peroxides, or acrylate metal salts in the polybutadiene rubber used for a core composition is known to increase C.O.R. However, this also leads to increased compression, resulting in poorer ball feel and increased driver spin rate, which results in reduced flight distance. This relationship between C.O.R. and compression can be adjusted only to a limited extent using known accelerators, cross-linking agents, and co-cross-linking agents.
In view of the above, it is apparent that improved golf ball cores that result in golf balls having optimal performance, e.g., spin rate value, hit-feel characteristics, and durability, while demonstrating ease of manufacture, as well as methods for making these cores are needed. The present invention fulfills these needs and provides further related advantages.